HYBRID VEHICLES FOR CLEAN CITIES

Dr Colin Jefferson, Hybrid Transport Technology Ltd, on 11 February 2003

It is forecast that the total number of vehicles in the world will treble by 2050, so the need for improved vehicles, particularly in urban areas, is obvious. Dr Jefferson is mostly involved with public service road vehicles – buses and trams – that have a poor image as means of transport at present in many towns.

He first noted the current level of pollution caused by vehicles and the regulations being introduced to reduce the level. Then he considered methods of reducing emissions: improved exhaust treatments; cleaner engines; clean fuels; higher efficiency engines; and braking energy recovery

Engine and exhaust improvements are in hand by the manufacturers. Clean fuels, such as low-sulphur diesel and natural gas, are becoming available more widely. Braking energy recovery is common for trams but could improve bus and car performance considerably, and is particularly valuable in start-stop situations, such as urban driving. As much as 60% of the braking energy can be recovered with a fuel consumption reduction of 30%. To use any energy recovery system requires the provision of an energy storage system; this is achieved with a hybrid vehicle.

A hybrid vehicle has a small engine running at as near a constant power output as possible to obtain maximum efficiency, and a storage system which receives energy by braking the vehicle (instead of using friction to do so) and from the engine when its full output is not needed to propel the vehicle.

The power source may be a conventional IC engine using diesel (including bio-diesel), petrol (including petrol / ethanol or methanol mixtures) or natural gas fuel; a gas turbine; a battery or a fuel cell.

The energy storage system may be a battery; a flywheel; an hydraulic accumulator; or an ultra-capacitor. Batteries do not store much energy for their weight and bulk; hydraulic accumulators are also not widely selected. Flywheels are the most common at present but ultra-capacitors may be the most efficient when finally developed. A 2,700 farad capacitor stores 2Wh per litre, and this may be improved. The CCM flywheel stores 4 kWh and produces 300 kW to drive the vehicle. It is mounted on gimbals in a frame about 1m x 1m x 1m and spins in a vacuum at 15,000 rpm. It has been tested to 100% overload and artificially caused to disintegrate, when it was reduced to a powder retained in the safety shield.

Trams, running steel wheels on steel rails, use less energy to propel them than buses with rubber tyres or tarmac, but both forms of public transport are being investigated for hybrid propulsion. Pictures of many different experimental and trial vehicles being tested in different parts of Europe were displayed.

The Oerlikon bus has been used in Ghent and Leopoldsville; it charges its flywheel at bus stops from overhead power connections and runs between stops on the stored energy.

The British Electric Railbus has been run on test in Bristol between the Industrial Museum and the SS Great Britain; it uses a steel flywheel spinning at 3000 rpm, re-charged at stops. The Greek city of Kalamata is installing a number of these vehicles and a new system of routes through the town.

The ELETRA bus uses a diesel engine and battery storage and the LIREX regional train a diesel engine and a flywheel. The most advanced concept combining a fuel cell running on hydrogen and an ultra-capacitor is demonstrated in the Volkswagen Bora HyPower car. Commercially available hybrid cars are available in the USA from Toyota, The Prius, and from Honda, the Insight. They may be on sale in the UK soon.

Dr Jefferson was pessimistic about the use of fuel cells for transport. There are problems with fuel distribution and storage if hydrogen is used and the cost is still high; but all the motor manufacturers are producing development models with forecasts of availability to the motorist ‘in the near future’.

He considered the flywheel – natural gas engine combination as most likely to be used in public transport in towns within the next decade.

Discussion

A number of question were asked, particularly concerning the ability of flywheel-operated buses and trams, like the British Electric Railbus, to cope with hilly terrain.

The maintenance of natural gas or bio-fuel IC engines was claimed to be high, but it may be that the gas turbine engine can cope with these fuels better.

Donald Lovell